Last week NPR reported that scientists now trace some of the rise of American obesity to the fear of fat. Beginning in the 1970s, nutritionists began warning Americans to consume less fat. This initiated the “low fat” and “fat free” crazes that still linger.

Yet, it now seems that people who followed the advice of nutritionists at the time — to eat less cheese, milk, and meat and more pasta, potatoes, and rice — were likely to get fatter, not skinnier. The closer a person stuck to the dietary guidelines, the more weight they would gain and, the more weight they gained, the more others would pressure them to stick to the dietary guidelines. The phrase “cruel irony” only begins to capture it.

The ad below, from 1959, is a peek into another era. Just a few years before the fear of fat began, the sugar industry was plausibly suggesting that eating more sugar was the best way to stay slim. This was industry association propaganda, but no doubt the potato and pasta industries contributed to the story in the ’70s just as the meat and dairy industries are in on it today.

The revision of our nutritional guidelines reminds us to be skeptical of the conventional wisdom. Moreover, it should inspire us all to check our tendency to judge others. We don’t have perfect knowledge that allows us perfect control over our bodies. Scientists are doing the best they can — and hopefully not taking too much funding from for-profit food industries — and individuals are restricted by whatever knowledge and resources they have.